The present invention relates to a compact short back focus imaging lens system with two lenses, especially to an imaging lens system for mini-cameras or mobile phones with image sensors such as CCD (Charge Coupled Device) or CMOS (Complementary Metal Oxide Semiconductor), which is formed by one positive lens and one negative lens and is with features of short back focal length, short overall (total) length and low cost.
The electronics available now become more compact and multifunctional. Most of them such as digital still cameras, PC (personal computer) cameras, network cameras and mobile phones, even personal digital assistants (PDA) are equipped with an imaging lens system. The imaging lens system not only requires good imaging quality but also needs compact volume (short overall length) and lower cost so as to match users' requirements.
There are various designs such as two lenses, three lenses, four lenses or five lenses of the imaging lens system applied to mini electronics. Yet while a compromise of resolution and cost, the two lenses is preferred. There are various structures of conventional two lenses imaging lens system and the difference among them or technical character is in the shape of the two lenses, location of the convex surface/concave surface, positive/negative refractive power, or relative optical parameters. Among these designs, the combination of a first lens with positive refractive power and a second lens with positive refractive power can achieve requirement of minimized volume, such as prior arts disclosed in US2005/0073753, US2004/0160680, U.S. Pat. No. 7,110,190, U.S. Pat. No. 7,088,528, EP1793252, EP1302801, JP2007-156031, JP2006-154517, JP2006-189586, TWM320680, TWI232325, and CN101046544 etc. However, the overall length of these imaging lens systems still requires further improvement to be reduced. For the user's requirement of short back focal length, the imaging lens systems disclosed in US2006/0221467, JP2005-121685 and JP2006-154517, use the combinations of a lens with positive refractive power and a lens with negative refractive power, the imaging lens system disclosed in US2003/0197956 uses the combinations of a lens with negative refractive power and a lens with positive refractive power, the imaging lens system disclosed in U.S. Pat. No. 5,835,288 is formed by the combinations of a biconcave lenses and a biconvex lenses, the imaging lens system disclosed in JP2004-0281830 uses the combinations of a lens with positive/negative refractive power and a lens with positive refractive power. Or the imaging lens systems disclosed in JP2003-215446, JP2004-177976, EP1793252, EP1793254, U.S. Pat. No. 6,876,500, US2004/0160680, U.S. Pat. No. 7,088,528, U.S. Pat. No. 7,492,528 , JP 3146386, TWI266074, use the combination of two lenses with positive refractive power so as to reduce the overall length. Therefore, in the different designs for reducing the overall length of the imaging lens system with two lenses, it is effective to design a short back focus imaging lens system. But the design with reducing the back focal length will cause the difficulty for adjusting the focal length of the imaging lens. Thus, the present invention provides a better design of the imaging lens system by reducing the back focal length as well as the air gaps between lenses so as to decrease the difficulty for adjusting the focal length and the distortion of image in order to let the imaging lens system be conveniently applied to mini cameras and camera phones.